1. Field of the Invention
The present invention generally relates to a support table, and more particularly to a support table for supporting a board which is used in the manufacture of an electronic module and to a screen printing method using the same.
2. Description of the Related Art
Electronic apparatus and semiconductor products are gradually progressing toward becoming faster, smaller, and thinner in order to meet the pressing demands for miniaturization, high integration density and high operational speed. Further, a module which has a plurality of electrically linked integrated circuits (ICs) installed on a printed circuit board (PCB) is used to meet the demand for an IC having a large memory capacity.
Such a PCB can be produced by either of the following two methods. In the first method, a plurality of semiconductor devices are mounted on a single board, and then the single board is divided into a plurality of individual boards. In the second method, the semiconductor devices are mounted on individual boards after they have been divided out from a single board. That is to say, the step of dividing a board into a plurality of individual boards and the step of mounting the semiconductor devices on the boards can be carried out in any order.
The modules are classified as single-sided modules and double-sided modules. The single-sided module has a plurality of semiconductor devices mounted on only one side of a board, while the double-sided module has a plurality of semiconductor devices mounted on both sides of a board.
Usually, after a lead material is applied to land patterns of the board, the semiconductor devices are mounted on the lead. A screen printing method or a dispenser method is used to form the leads.
In the dispenser method, a syringe containing the lead material is compressed by air, whereby the lead material is ejected through a nozzle ofthe syringe. This method has an advantage in that it is not affected much by the lack of planarity of the board because the lead material is ejected though the nozzle. Also, the amount of the lead material emitted can be controlled by modifying the pressure of the compressed air, the emission period, the interior diameter of the nozzle, and the viscosity (centipoise) of the lead material. However, this method is not suitable for a fine lead-application step because it can not meet the requirements of mass-productivity or those for the manufacture of a multi-pin semiconductor device package.
On the other hand, the screen printing method has an advantage in that the lead material can be applied to the board all at once. In this method, a screen in which hole patterns are formed is in contact with part of the board. However, this method requires the board to be precisely planar, because the lead material is applied to the board which is in contact with the screen. The screen which is used in the screen print method can be either a mesh screen or a metal screen (also referred to as "a metal mask").
The hole patterns of the mesh screen are formed by applying an emulsion on a mesh of stainless wire, nylon, or tetron. The mesh has 80 to 100 hole patterns and such a mesh screen can advantageously be employed in connection with the manufacture of large boards. The thickness of the printed pattern depends on the thickness of the lead and the toughness or durability of the mesh.
The metal mask is expensive compared to the mesh screen, but has an advantage of high durability. Because finer hole patterns can be formed in the metal mask, it has an advantage in that it can apply leads on finer land patterns formed on the board. However, it has a disadvantage in that the restricted size of the metal mask limits the area in which printing can be carried out on the board.
Furthermore, in the lead application step using the screen, because a squeegee applies pressure to the board, a support table for maintaining the board planar and supporting the board is required.
FIG. 1 depicts the screen printing of leads on one surface of a board which is resting on a conventional support table and FIG. 2 is a cross-sectional view taken along line 2--2 in FIG. 1.
Referring to FIG. 1 and FIG. 2, a conventional support table 10 for supporting a module board 20 has a planar top surface that receives a bottom surface of the board. The support table 10 prevents the board 20 from warping when the lead material 50 is applied to the board 20 during the screen printing step. Magnets 73 are attached to a bottom surface of the support table 10.
The board 20, which is devoid of semiconductor devices, is loaded on the support table 10. A screen 30 such as mesh or metal mask is disposed on the top surface of the board 20 and the creamy lead material 50 is applied to the board 20 by a squeegee 40. At this time, since the bottom surface of the board 20 is supported by the support table 10, the board 20 is prevented from warping.
The support table 10 is fixed to one surface of a driving device (not shown). The support table 10 is elevated by the driving device until the support table nears the screen 30 whereupon the lead material is applied to land patterns 24 already formed on the board 20. The support table 10 is fixed to the driving device by the magnets 73. The support table 10 is made of a non-magnetic material such as aluminum alloy.
A PCB (printed circuit board) is mainly used as the board 20. Generally, the board 20 is manufactured as a single body. Hole patterns 32 are formed in the screen 30. The screen 30 is placed on the board 20 with the hole patterns 32 aligned with the land patterns 24. The lead material 50 is applied onto the land patterns 24 of the board 20 through the hole patterns 32. Accordingly, after semiconductor devices are mounted to the leads, the board 20 is cut along scribing holes 26 to separate it into individual boards. In the present embodiment, land patterns 24 on which six (6) semiconductor devices 60 (FIG. 3) are mounted are formed per individual separated board 20. The semiconductor devices 60 are mounted on the resulting leads using a reflow solder process. The land patterns which receive other electronic components, such as capacitors or resistors, are omitted from the drawings for the sake of simplicity.
The mounting of semiconductor devices on the other surface of the board will now be described. FIG. 3 depicts the screen printing of leads on the other surface of the board, after semiconductor devices have been mounted on one surface of the board. FIG. 4 is a cross-sectional view of a double-sided module in which the semiconductor devices have been mounted on the other surface of the board.
Referring to FIG. 3 and FIG. 4, because the semiconductor devices 60 which are mounted on one surface of the board 20 have the same height, the support table 10 which is in contact with the semiconductor devices 60 has a planar top surface. That is to say, the support table 10 has a planar top surface which supports the semiconductor devices 60 having the same height. The bottom surfaces of the semiconductor devices 60 are in contact with the top surface of the support table 10. The support table 10 shown in FIG. 1 and FIG. 2 can be used in this case.
The lead material is applied to the other surface of the board 20 by the same process described in connection with FIG. 1 and FIG. 2, and the semiconductor devices 60 are mounted on the resulting leads by the reflow soldering process.
In the above-described embodiment, semiconductor devices having the same height were mounted on the board 20. However, as shown in FIG. 5 and FIG. 6, semiconductor devices having different heights also can be mounted on the board 20 to produce a double-sided module.
Referring to FIG. 5 and FIG. 6, semiconductor devices 160, 165 which are mounted on one surface of a board 120 have different heights. The semiconductor devices 160 which are disposed to both sides of the board 20 have a greater height and are denoted as `high semiconductor devices`, whereas the semiconductor devices 165 which are disposed at the center of the board 120 have a smaller height and are denoted as `low semiconductor devices`. A support table 110 has a projection 115 at the center thereof and on which the low semiconductor devices 165 are supported. Because the height of the low semiconductor devices 165 is less than that of the high semiconductor devices 160, the projection 115 is formed so as to compensate for the difference between the height of the high semiconductor devices 160 and the height of the low semiconductor devices 165. The height of the projection 115 is the same as the difference of the height between the high semiconductor devices 160 and the low semiconductor devices 165. The magnet 173 is attached to the bottom surface of the support table 110.
The projection 115 prevents the board 120 from warping because the support table 110 directly supports all of the semiconductor devices having different heights.
Note, although semiconductor devices 160, 165 having different heights were mounted on both surfaces of the board 120, semiconductor devices having the same height can also be mounted on one surface of the board 120 to produce a double-sided module.
As an example, the heights of semiconductor devices which are mounted on the board 120 are as follows:
SOJ (Small Outline Package)--about 3.6 mm; PA1 TSOP (Thin Small Outline Package)--about 1.2 mm ; and PA1 QFP (Qu ad Flat Package)--about 3.0 mm.
In manufacturing the above-described double-sided module, in the case of mounting semiconductor devices having the same height, the support table which has a planar top surface as shown in FIG. 1 and FIG. 2 is used. However, in the case of mounting semiconductor devices having different heights, the support table which has the projection as shown in FIG. 5 and FIG. 6 is used.
The prior art is problematic in that a support table must be newly produced when different semiconductor devices of different heights are to be mounted on the board. The cost for manufacturing a support table is very high. Therefore, this high cost becomes a significant part of the cost of producing the modules.